I created a problem on my own and now stuck in troubleshooting the issue.

Situation:
Control Panel buttons A-G and the 8 buttons left (PROG/COMBI etc), are not working. All other buttons working. It is not the "button" broken or stuck issue, as the buttons electronical do work and clearly creating a contact (measured with multitool).

What I did (not sure if that is anyhow related).

I resoldered a few buttons, but it still worked after that.
I changed the display to a new LED backlight display. This works. It also worked after simply plugging in the new Display. However it was to bright, as expected as the contrast ratio is not correct anymore.

I then replaced R149 from 220kO to 180kO (as defined in the manual for the display).

Then I plug everything back in place and since then the said buttons do not work anymore.

The display hack indeed is working! ...

Looking at the diagrams in the Service Manual, it looks like line 12 and 14 of the connection from panel to main pcb is somehow the issue, as that is the only reason, why it would not work anymore.

however ... all lines in the cable working and I also cannot identify any button not working, or any other shortage. All diodes on the panel pcb are working as well ...

I also scratching my head if the Display hack could be the root cause) .. But cannot see any point how this could be related looking on the cirquit diagrams.

Any Idea how to narrow down the issue? How can I measure e.g. if the issue is on main PCB or on panel PCB?

Hmmm, from what I can gather here you swapped out the resistor on a board? I've done 4 of these replacements, 3 on my 01R/W's and 1 on my 01W/FD. In every case I removed the EL Foil inverter off the power board and used the +5v from there to power the LED display backlight.

The big difference being I used an INLINE resistor on the +5v wire to the display. I've never had a problem, contrast works perfectly. I can't say for sure if your resistor swap out has introduced the problem. If it were me I would check the traces on the panel to make sure none have been broken or shorted out. Once I'm sure that is OK I would start to retrace my steps and consider putting back the old resistor to rule that out.

Ahhh, can I assume (I read your post again) you purchased a pre-wired display from eBay and it derives it's power from the pin header rather than using power directly from the power supply like my mods?

The question for me would then be what else is affected by changing that resistance?